The invention relates to a device for optically reading record carriers of a first type and record carriers of a second type, each type of information carrier having an information structure, the information structure of the record carrier of the first type being adapted to be read with radiation of a first wavelength and the information structure of the record carrier of the second type being a phase structure arranged in tracks and adapted to be read with radiation of a second wavelength longer than the first wavelength, (as used hereinafter in the specification and claims, phase structure means a structure which causes radiation received from a scanned portion of the track to exhibit variations due to optical interference) device comprises scanning means for scanning the information structure in a track of a record carrier of the first or the second type present in the device by means of a focused beam of radiation of the first wavelength, a detection system for converting radiation received from a scanned portion of the track into an analog read signal, and a signal converter for converting the read signal into a corresponding binary information signal.
Such a device is known from EP-A-0,525,896, to which U.S. Pat. No. 5,235,583 corresponds. By means of the known device it is possible to read high-density record carriers comprising an information structure which is arranged in the form of tracks and whose dimensions are adapted to be read by means of a beam of radiation having a short wavelength (of the order of magnitude of 400 to 600 nm). In the known device the same radiation beam can be used for reading record carriers with a lower information density, such as for example Compact Discs, on which the recorded information is represented by a phase structure arranged in tracks in the form of a pattern of pits whose geometry is adapted to be read with a beam of radiation of a wavelength larger than that of the radiation beam used in the device, for example of the order of magnitude of 800 nm. In order to ensure a reliable read-out when the record carrier with the lower radiation density is read the radiation beam is not aimed at the centre of the track, as is customary, but at the edge of the track. To read the high-density record carrier the radiation beam is aimed, as is customary, at the centre of the track. This means that depending on the type of record carrier the tracking system is set to a first mode, in which the centre of a track is followed, or to a second mode, in which the edge of a track is followed. Operating in two different tracking modes is undesirable. This is particularly so in the case that a so-called three-spot tracking system (having one main spot and two satellite spots) is used. This is because for a given position of the satellite spots relative to the main spot it is then only possible to realise a tracking system for a very limited number of track pitch and track width combinations on the record carriers.